An optoelectronic component (e.g. an organic light emitting diode (OLED), for example a white organic light emitting diode (WOLED), a solar cell, etc.) on an organic basis is usually distinguished by its mechanical flexibility and moderate production conditions. Compared with a component composed of inorganic materials, an optoelectronic component on an organic basis can be produced potentially cost-effectively on account of the possibility of large-area production methods (e.g. roll-to-roll production methods).
A WOLED consists e.g. of an anode and a cathode with a functional layer system therebetween. The functional layer system consists of one or a plurality of emitter layer/s, in which the light is generated, one or a plurality of charge generating layer structure/s each composed of two or more charge generating layers (CGL) for charge separation, and one or a plurality of electron blocking layers, also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layers, also designated as electron transport layer(s) (ETL), in order to direct the current flow.
The luminance of OLEDs is limited, inter alia, by the maximum current density which can flow through the diode. In order to increase the luminance of OLEDs, it is known to combine one or a plurality of OLEDs one on top of another in series (so-called stacked or tandem OLED). By means of stacking one above another, it is possible to obtain significantly longer lifetimes in the OLED with practically the same efficiency and identical luminance, whereas with the same current density it is possible to realize N times the luminance in the case of N-OLED units. In this case, the layers at which the OLED units contact one another are accorded particular importance. An electron-conducting region of one diode and a hole-conducting region of the other diode meet at these layers. The layers between these regions, the so-called charge generating layer (CGL) structure, should be able to separate electron-hole pairs from one another and to inject electrons and holes into the OLED units in opposite directions. This enables the continuous charge transport through the OLED series circuit.
For the stacking one above another, charge generating layers consisting of a highly doped pn junction are therefore required.
In the simplest embodiment, the charge generating layer structure conventionally consists of a hole-conducting charge generating layer and a first electron-conducting charge generating layer, which are directly connected to one another, with the result that illustratively a pn junction is formed. This generates a potential jump in the pn junction or a built-in voltage.
The potential jump or the built-in voltage can be influenced by means of the work function, the doping of the layers, and also the formation of interface dipoles at the pn junction by means of the substances used.
In the pn junction, a space charge zone is formed, in which electrons of the hole-conducting charge generating layer tunnel into the first electron-conducting charge generating layer. Often the first charge generating layer is physically connected to a second charge generating layer, wherein the second electron-conducting charge generating layer is often an n-doped charge generating layer.
As a result of a voltage being applied across the pn junction in the reverse direction, in the space charge zone electrons and holes are generated which migrate into the emitter layers of the OLED units and can generate electromagnetic radiation as a result of recombination (e.g. light).
The hole-conducting charge generating layer and the electron-conducting charge generating layers can each consist of one or a plurality of organic and/or inorganic substance(s) (matrix).
In the production of the charge generating layer, the respective matrix is usually admixed with one or a plurality of organic or inorganic substances (dopants) in order to increase the conductivity of the matrix and in order to carry out potential matching or energy level matching. This doping can produce electrons (n-doped; dopants e.g. metals having a low work function, e.g. Na, Ca, Cs, Li, Mg or compounds thereof, e.g. Cs2CO3, Cs3PO4, or organic dopants from the company NOVALED, e.g. NDN-1, NDN-26) or holes (p-doped; dopant e.g. transition metal oxides, e.g. MoOx, WOx, VOx, organic compounds, e.g. Cu(I)pFBz, F4-TCNQ, or organic dopants from the company NOVALED, e.g. NDP-2, NDP-9) as charge carriers in the matrix.
As substance of the hole-conducting charge generating layer above or on the first electron-conducting charge generating layer, use is usually made of an undoped organic substance as hole transport conductor (hole transport layer HTL), e.g. αNPD.
Furthermore, undoped hole-conducting charge generating layers are known which include a transparent metal oxide as hole-conducting substance, for example WO3 or MoO3.
A prerequisite for the use of a charge generating layers in an optoelectronic component are a simple construction, i.e. as few layers as possible which can be produced as easily as possible. Furthermore, a low voltage drop across the charge generating layers and a highest possible transmission of the charge generating layers are required, i.e. the lowest possible absorption losses in the spectral range of the electromagnetic radiation emitted by the OLED.
In the context of this description, an organic substance can be understood to mean a carbon compound which, regardless of the respective state of matter, is present in chemically uniform form and is characterized by characteristic physical and chemical properties. Furthermore, in the context of this description, an inorganic substance can be understood to mean a compound which, regardless of the respective state of matter, is present in chemically uniform form and is characterized by characteristic physical and chemical properties, without carbon or a simple carbon compound. In the context of this description, an organic-inorganic substance (hybrid substance) can be understood to mean a compound which, regardless of the respective state of matter, is present in chemically uniform form and is characterized by characteristic physical and chemical properties, including compound portions which contain carbon and are free of carbon. In the context of this description, the term “substance” encompasses all abovementioned substances, for example an organic substance, an inorganic substance, and/or a hybrid substance. Furthermore, in the context of this description, a substance mixture can be understood to mean something which has constituents consisting of two or more different substances, the constituents of which are very finely dispersed, for example. A substance class should be understood to mean a substance or a substance mixture composed of one or a plurality of organic substance(s), one or a plurality of inorganic substance(s) or one or a plurality of hybrid substance(s). The term “material” can be used synonymously with the term “substance”.
In the context of this description, a hole-conducting charge generating layer can also be designed or understood as a hole transport layer.
In various embodiments, an electron-conducting layer of an electronic component can be understood to mean a layer in which the chemical potential of the substance or of the substance mixture of the layer is energetically closer to the conduction band than to the valence band, and in which more than half of the freely mobile charge carriers are electrons.
In various embodiments, a hole-conducting layer of an electronic component can be understood to mean a layer in which the chemical potential of the substance or of the substance mixture of the layer is energetically closer to the valence band than to the conduction band and in which more than half of the freely mobile charge carriers are holes, i.e. free orbital sites for electrons.
Unlike in the case of purely inorganic layers in semiconductor components, the molecules of organic layers can partially diffuse into other organic layers (partial layer interdiffusion), for example parts of an organic, first electron-conducting charge generating layer (e.g. HAT-CN) into an organic hole-conducting charge generating layer hole transport layer (e.g. αNPD).
When an electric field is applied to the charge generating layer structure, an additional drop of the operating voltage (and thus of the electrical power) across said layer structure is measureable by means of the layer interdiffusion. This voltage drop cannot be used for the generation of light and thus reduces the efficiency of the stacked OLEDs.
The additional voltage drop can increase with the operating period, since the diffusion of conductive molecules is directed in an electric field. This limits the operating period of organic optoelectronic components.
A further disadvantage when using organic hole-conducting charge generating layer is the low charge carrier density thereof and the relatively weak interface dipoles. The low charge carrier density leads to a higher voltage drop across said layer, i.e. the layer has a lower electrical conductivity. The often weak interface dipoles make it more difficult to separate hole and electron at the interface of the hole-conducting charge generating layer and the first electron-conducting charge generating layer.
Furthermore, organic hole-conducting charge generating layers, for example αNPD, can be thermally sensitive. The substance of the organic hole-conducting charge generating layer can for example start to crystallize, for example at temperatures of approximately 95° C. in the case of αNPD. By means of the crystallization of the substance of the organic hole-conducting charge generating layers, the layer can lose its functionalities in the charge generating layer structure, such that the optoelectronic component can become unusable.
A hole-conducting charge generating layer composed of an inorganic substance could solve the problem of the layer interdiffusion, the low conductivity, the low charge carrier separation and the temperature sensitivity. The formation of hole-conducting charge generating layers from inorganic substances has not been possible to be realized hitherto for a number of reasons. In this regard, in the case of many known inorganic substances, the electrical properties are incompatible with the electrical properties of the organic first electron-conducting charge generating layer. The work function of the inorganic substances is too high (greater than approximately 3 eV) and/or the energy of the valence band is less than the energy of the conduction band of the first electron-conducting charge generating layer in physical contact with the hole-conducting charge generating layer.
A further obstacle is presented by the generating conditions of the hole-conducting charge generating layer composed of an inorganic substance. Inorganic substances for forming the hole-conducting charge generating layer are unsuitable if the hole-conducting charge generating layer can be formed only by production conditions which are incompatible with organic layers, e.g. temperature >>100° C.
What is furthermore disadvantageous in the choice of inorganic substances for the hole-conducting charge generating layer is the optical properties thereof, e.g. the transmission. Many inorganic substances exhibit absorption in the wavelength range of between approximately 400 and approximately 650 nm and are therefore not transparent. The efficiency of the optoelectronic component is reduced as a result. For these reasons, only compromises have been able to be achieved for the transition of the hole-conducting charge generating layer with the first electron-conducting charge generating layer with electron-conductive metal oxide semiconductors.